JP2985767B2 - Surface-treated steel sheet with excellent corrosion resistance after processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet having excellent corrosion resistance after processing, particularly to a fuel tank for automobiles and motorcycles, which exhibits high corrosion resistance to fuels such as gasoline and gasohol, and also to a stove and a boiler. The present invention relates to a surface-treated steel sheet suitable for applications such as a kerosene tank, an oil filter requiring extremely strict workability and high corrosion resistance after processing.

[0002]

2. Description of the Related Art Materials for fuel tanks such as automobiles and motorcycles have not only weldability but also general corrosion resistance on the outer surface (hereinafter referred to as outer corrosion resistance), and the inner surface has fuel corrosion resistance against fuel such as gasoline. Is required. These are collectively called corrosion resistance or corrosion resistance after processing. Conventionally, turn sheets (10 to 25% Sn-Pb alloy plated steel sheets) have been widely used as fuel tank materials. However, Pb in the plating film is harmful to the human body, the plating film is easily dissolved in the alcohol oxide when alcohol-containing fuel is used, and pinholes in the plating film are inevitable.
There is a problem in that Fe that is more base than the plating film is preferentially corroded from the pinholes, resulting in insufficient perforation corrosion resistance. Therefore, alternative materials have been demanded.

In particular, in recent years, gasoline / alcohol mixed fuels (such as M15 containing about 15% methanol, M85 containing about 85% by weight of methanol, etc.) are called gasohol due to emission regulations in consideration of environmental issues. The use of representative alcohol-containing fuels is being promoted in some countries. However, since the conventional turn sheet is easily corroded by the alcohol-containing fuel as described above, there is an urgent need to develop a fuel tank material having excellent fuel corrosion resistance to the alcohol-containing fuel.

[0004] From this point of view, application of a Zn-Ni alloy electroplated steel sheet to a fuel tank has been conventionally considered in consideration of corrosion resistance after processing and cost. The following publications can be cited as prior art.

JP-A-58-45396 discloses that the Ni content is 5%.
A surface-treated steel plate for a fuel tank is shown, in which a chromate treatment is applied on a Zn-Ni alloy electroplating having a thickness of 0.5 to 20 μm and a thickness of 0.5 to 20 μm.

JP-A-5-106058 discloses that the Ni content is 8
A surface-treated steel sheet for a fuel tank, which is provided with a Zn—Ni alloy plating of 2020 wt% at an adhesion amount of 10-60 g / m ² and subjected to a chromate treatment, is shown.

[0007] These surface-treated steel sheets have very good external corrosion resistance, but their fuel corrosion resistance, especially after processing, is still not sufficient. For example, salt water is mixed in alcohol-containing fuel. Corrosion was likely to occur in very harsh environments such as cases. Further, when the chromate film or the plating film is thickened to improve this, there is a problem that weldability, which is an important performance as a material for a fuel tank, is deteriorated.

As a matter of course, techniques for providing cracks in a plating layer are disclosed in JP-A-5-25679 and JP-A-4-33.
Although these are disclosed in Japanese Patent No. 7099, these are intended to improve the adhesion of the upper plating layer by providing an additional plating layer on the underlying plating layer provided with cracks by the anchor effect. This is intended to improve the impact adhesion resistance when used for an outer panel of an automobile.

Furthermore, Japanese Patent Application Laid-Open No. 62-297490 discloses that a crack is formed in a Ni alloy plating layer.
Thereby, a fine uneven pattern is formed on the surface to realize blackening.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art of Zn-Ni alloy plating + chromate surface-treated steel sheet. The purpose is to develop a technology that can improve the fuel corrosion resistance after processing without impairing the weldability.

[0011]

Means for Solving the Problems The present inventors have conducted various studies in order to solve such problems, and as a result, have found that a continuous Zn-X (X = Ni, Co, Mn, Cr) using an acidic bath. Kind of or two or more types, hereinafter referred to as X) The energization is stopped at the final stage of the alloy electroplating process, immersion is performed for a short time in a plating solution, and chromate treatment is performed thereon. It was noticed that the fuel corrosion resistance was significantly improved. As a result of investigating the cause, immersion in this acidic plating solution caused cracks in the Zn-X alloy plating layer, and the crack density, maximum crack width, and crack depth on the surface of the plating film thus generated were identified. , The fuel corrosion resistance after processing is remarkably improved. In addition, by pre-plating or pre-Ni alloy plating on the base material of this steel sheet, cracks that have reached the base material can be suppressed on the pre-plating, further improving The present inventors have found that the fuel corrosion resistance after processing is improved, and completed the present invention.

[0012] Here, the present invention has a plating layer containing 70 wt% or more of Ni having a plating adhesion amount of 0.001 to 5 g / m ^{2 on} one side as a first layer of a plating film, and a second layer comprising: Alloy element X is Ni: 3 to 18 wt%, Co: 0.02 to 3 wt%, Mn: 25
At least one selected from the group consisting of ~ 45wt% and Cr: 8 ~ 20wt%, and the coating weight per side is 5 ~ 50g /
has a Zn-X alloy plating film is m ^2, and more the Zn-
10 to 200m in equivalent amount of metal Cr on X alloy plating film
It is made of a surface-treated steel sheet with a g / m ² chromate film,
The Zn-X alloy plating film under the chromate film has cracks, and the density of these cracks is 1 mm × 1 m on the plating surface.
expressed as the number of areas surrounded by cracks in the field of view of m
90% or more of the cracks are in the range of 1000 to 150,000 and the maximum width of the crack is 0.5 μm or less, and the crack depth is 80% or more of the plating thickness of 80% or more. This is a surface-treated steel sheet having excellent corrosion resistance after processing.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the accompanying drawings is a schematic explanatory view showing a cross-sectional structure of a plating film of a surface-treated steel sheet according to the present invention. , A Zn-X alloy plating film 2 is provided thereon, and a chromate film 3 is provided thereon, and a crack 4 is provided on the plating film 2.

According to the present invention, the first layer (pre-plating) of the plating film contains 70% by weight or more of Ni, and the plating adhesion amount on one side is 0.001 to 5 g / m ² . The purpose of providing this pre-plating film is to prevent cracks generated in the plating of the second layer from reaching the base material and to stop immediately above the first layer (pre-plating). Is to be further improved.

Since Ni is a metal nobler than Fe, it is hardly oxidized, and has an effect of preventing oxidation of the surface of Fe by plating with a small amount of adhesion. Therefore, the second causing cracks
It is effective to use it as a pre-plating layer. By performing this pre-plating, cracks generated in the plating film of the second layer do not reach the base material directly, and the Fe base material around the base material is protected by Ni, so that the corrosion resistance after processing is dramatically improved. To improve.

The method of plating the first layer is as follows.
Electroplating or displacement plating (electrically immersed immersion plating) is preferred from the viewpoint of the amount of coating, but Ni-containing liquid or solid (
(Paste). The composition is Ni
Is contained, the composition of the remaining 30 wt% is not particularly defined. For example, a Fe group metal element such as Fe or Co, or a transition or non-transition metal element such as Zn, Cr, Mn, Cu, or Al may be used. An element that precipitates may be used. Further, an organic substance or an oxide containing C, H, O, N, P, S, and other elements may be included.

In order to sufficiently produce the effect of pre-plating, the Ni content in the first layer (pre-plating) must be 70%.
wt% or more, and the coating weight of the first layer is 0.001 to 5 g /
m ² or more. If the Ni content is less than 70 wt%, Ni
It is difficult to obtain the excellent oxidation resistance possessed by. If the amount of adhesion is the total amount of the first layer (pre-plating) on one side, less than 0.001 g / m ² will not provide the effect of improving corrosion resistance after processing of the second layer, and if it exceeds 5 g / m ^2, it will be hard and brittle Ni alloy Due to the characteristics of the plating, the workability is lowered and the production cost is increased, which is not preferable. Preferably adhesion amount is 0.005 ~0.1g / m ^2.

The second layer is composed of a Zn-X alloy plating film, and the alloy composition is such that X is Ni: 3 to 18% by weight, Co: 0.02%.
At least one selected from the group consisting of 〜3 wt%, Mn: 25-45 wt%, and Cr: 8-20 wt%. However, when X includes two or more alloying elements, preferably, the second and subsequent elements are Ni: 3 to 18 wt%, C
o: may be selected from the group consisting of 0.02 to 3 wt%, Mn: 25 to 45 wt%, and Cr: 8 to 20 wt%, or the second and subsequent elements are selected from Ni, Co, Mn, and Cr, and the total amount thereof May be limited to 5% or less.

The X content of the plating film means not the X content immediately after the electroplating of the Zn—X alloy, but the average value of the X content after cracking of the plating film. In the present specification, this is simply referred to as X content.

The X content of the entire plating film is such that when X alone is added, when each is too lower than the above range, both the outer corrosion resistance and the fuel corrosion resistance after processing are not sufficient. If the amount is higher than the above range, including the case where two or more kinds of X are added, the workability or the outer surface corrosion resistance becomes insufficient.

In particular, the second and subsequent elements added in a total amount of 5% or less are added for further improvement of the outer surface corrosion resistance. When the total amount exceeds 5%, workability is slightly deteriorated. In the case of Ni alone, the content is preferably 3 to 14
% Or 9-18%, more preferably 10-14%, even more preferably 11-13%.

If the coating weight (amount per one side, the same applies hereinafter) is less than 5 g / m ² , the corrosion resistance after processing is insufficient, while if it is more than 50 g / m ² , the realized performance is poor. It is saturated and uneconomical, and the weldability deteriorates. The coating weight is preferably 7 to 30 g / m ² , more preferably 10 to 25 g / m ²
It is.

According to the present invention, as described above, cracks are generated in the Zn—X alloy plating film at a density in the range of 1000 to 150,000 / mm ² , and a chromate treatment is performed thereon.
Fuel corrosion resistance after processing is dramatically improved. Although the reason is not necessarily clear, the incorporation of chromate into such cracks increases the anchor effect of the chromate film being firmly fixed and the surface area covered by the chromate film having excellent corrosion resistance to the cracks. In non-cracked Zn-X alloy plated steel sheets, cracks occur in the plating film during press working, and the corrosion resistance is deteriorated by exposing the base steel sheet.On the other hand, cracks are generated in the plating film in advance, and the cracks are generated. By covering with a chromate film, it is conceivable that cracks newly generated at the time of press working are small and the corrosion resistance as a whole is improved.

In the present invention, the crack density is represented by the number of areas surrounded by cracks in a visual field of 1 mm × 1 mm on the plating film surface. This crack density measurement is in a 0.1 mm x 0.1 mm field of view that randomly takes 30 SEM (scanning electron microscope) photographs of the plating film surface of the sample at a magnification of 1000 times and sets each photograph at random. This is performed by counting the number of areas surrounded by cracks (the number of cracks) by image analysis. The average value of the number of cracks obtained from 30 photographs is calculated, and the value multiplied by 100 is defined as the crack density. The “region surrounded by cracks” is, as schematically shown in FIG. 2, a region which is seen in an SEM photograph and is divided into islands by cracks 4.

According to the present invention, Z is determined so that the crack density determined in this way is not less than 1000 and not more than 150,000.
By generating cracks on the surface of the nx alloy plating film, for example, the corrosion resistance against corrosion by gasoline or gasohol, that is, the fuel corrosion resistance after processing is remarkably improved. If the crack density is more than 150,000, the number of cracks is too large, and the plating coverage becomes too small, so that the fuel corrosion resistance after processing deteriorates. If the crack density is less than 1,000, the effect of improving the fuel corrosion resistance after processing deteriorates. Preferably, the crack density is
1000 to 50,000.

It is assumed that 90% or more of the cracks have a maximum width of 0.5 μm or less. The maximum width of the crack is a value obtained by measuring the largest crack width among the cracks existing in the visual field of 0.1 mm × 0.1 mm of the above 30 SEM photographs. That is, 1 for each photo
The individual field of view is selected, and the maximum width of each field is defined as the maximum width of the area, and the ratio of those having a maximum width of 0.5 μm or less is determined. If the maximum crack width is 0.5 μm or less and the ratio is out of the range of 90% or more, the environmental barrier effect of the plating film is impaired, and both the corrosion resistance after processing and the fuel corrosion resistance deteriorate. Preferably, the maximum width of the cracks 0.4
90% or more of particles having a diameter of μm or less exist.

The crack depth was obtained by taking an SEM photograph of a cross section at a magnification of 2000 times within a range of 1 mm in length, measuring the crack depth in this range, and comparing this with the plating thickness. It is assumed that cracks having a depth of 80% or more of the plating thickness are present in 80% or more of the total number of cracks in this visual field. Within this range, the outer corrosion resistance and the fuel corrosion resistance after processing are excellent. If the crack depth is shallow, such as less than 80% of the plating thickness, or if the proportion of cracks that are 80% or more of the plating thickness is less than 80%, cracks will be newly formed during processing, and the outer surface after processing Corrosion resistance and fuel corrosion resistance are impaired.

Preferably, the crack density is from 1,000 to 50,000
90 cracks with a maximum crack width of 0.4 μm or less
% Or more, and the crack depth is 90% or more of the plating thickness, which means that 95% or more of the total number of cracks is present.

The method for generating the cracks on the surface of the Zn—X alloy plating film is not particularly limited, and a mechanical method by performing plastic working such as bending back or tension after the plating treatment is also possible. Chemical treatment by etching with an acid or alkali aqueous solution is preferable because it is excellent in control of crack density and uniformity of cracks. In order to control the crack density or the like within the above range, for example, the immersion conditions, particularly the time, may be adjusted.

When the Zn-X alloy electroplating is performed in an acidic bath (eg, a sulfate bath), the acidic plating solution can be used for etching. That is, in the electroplating process in which a steel sheet is energized in an acidic bath to perform Zn-X alloy plating, as described above, the energization is stopped at the final stage of plating, and the steel sheet is immersed in a plating solution in a non-energized state. By doing so, the plating surface can be etched and cracks can be generated. As a result, the post-plating etching is performed using the conventional plating equipment and plating solution without using a separate processing tank or acid or alkali aqueous solution prepared for etching, and necessary cracks are generated on the plating surface. Thus, the surface-treated steel sheet of the present invention can be manufactured efficiently without reducing the cost and increasing the number of steps. Of course, the immersion treatment of the plating solution can be performed in an immersion tank provided separately from the plating bath.

When the surface-treated plated steel sheet according to the present invention is used, for example, as a material for a fuel tank, the plating film corresponding to the inner surface of the tank is preferably immersed in an acidic plating solution as described above. Cracks are generated as specified in (1), but it is preferable that the plating film on the outer surface of the tank is treated in the same manner to generate cracks in the same manner as the inner surface. Thereby, in addition to the improvement of the fuel corrosion resistance of the tank inner surface, the corrosion resistance of the tank outer surface is also significantly improved. Also, as a practical matter, performing an etching treatment by immersion in an acidic plating solution or the like on only one side of a plated steel sheet requires a step of sealing or the like, and the operation becomes complicated. It is also advantageous from above.

In the present invention, after forming each such plating film, at least a surface which is used without being painted and needs corrosion resistance after processing is subjected to a chromate treatment, and a chromate film is formed on the plating film. The cracks in the plating film are covered with a chromate film. Even when painted and used, the cracks in the plating film are covered with a chromate film,
If the surface is coated with a paint film, the corrosion resistance of the outer surface is dramatically improved. Therefore, the outer surface may be subjected to a chromate treatment.

In the present invention, the chromate film is made of metallic Cr
It is formed so that the conversion amount becomes 10 to 200 mg / m ² . If the amount is less than 10 mg / m ² , the corrosion resistance after processing is not sufficiently exhibited, while if it exceeds 200 mg / m ² , weldability such as seam weldability deteriorates. The preferable coating amount of the chromate film is 50 to 180 mg / m ² in terms of metal Cr.

This chromate film may be any of a coating type, an electrolytic type and a reactive type. If a large amount of hygroscopic Cr ⁶⁺ is contained in the chromate film, the moisture in the fuel is adsorbed and fixed on the surface of the chromate film, and the portion may be partially corroded. Therefore, Cr in the chromate film
It is preferable that the proportion of ⁶⁺ is as small as possible. In that sense, it is desirable that the amount of Cr ^{6+ be} 5% or less of the total amount of Cr.

In another preferred embodiment of the present invention, in order to further enhance the corrosion resistance of the chromate film, silica is contained in the film in a SiO ₂ / Cr weight ratio of 1.0 to 10.0. If the weight ratio is less than 1.0, the effect of improving the corrosion resistance of the chromate film is insufficient, and if it exceeds 10.0, the stability of the chromate solution may be degraded and the operation may be adversely affected. May deteriorate. Preferably, the SiO ₂ / Cr weight ratio is from 1.5 to 9.5.

With regard to the silica species used in the present invention, dry-process silica (gas-phase silica or fumed silica) having a low water absorption is better than wet-process silica (colloidal silica or silica sol). Even when the chromate film contains silica, the metal of the chromate film
The adhesion amount in terms of Cr may be the same as described above.

[0037]

The present invention will be described below in detail with reference to examples.

Preparation of Surface-treated Steel Sheet Sample A 0.8 mm thick cold-rolled steel sheet equivalent to JIS SPCE was plated on both sides under the following conditions, and then Zn-coated in a sulfate bath under the following conditions. X-alloy electroplating is applied to both sides, and using this plating bath as it is, the resulting plated steel sheet is immersed in an acidic plating solution without electricity to etch the plating film on both sides, and the Zn-X alloy plating is performed. Cracks were introduced into the coating. The crack density, maximum crack width, and crack depth were adjusted by changing the immersion time in the plating solution. When a Zn-X alloy plating film having a low crack density and a large maximum crack width was required, biaxial tension was applied to the plated steel sheet after etching. The crack density, maximum crack width, and crack depth of the etched plating surface were determined by SEM as described above.
I got it from the photo.

[Pre-plating conditions] Electroplating: Plating bath composition: Ni 0.01 to 0.1 mol / L Other components 0.0001 to 0.1 mol / L (Fe, Co, Zn) Other ions SO ₄ ^2- , NH ₄ ⁺ pH 4.5 ~ 6.5 (adjusted with sulfuric acid and ammonia) Plating condition: Bath temperature 30 ~ 40 ℃ Current density 2 ~ 8 A / dm ² solution flow rate 0.06 ~ 1.40 m / s Displacement plating: Plating bath composition: Ni 0.01 ~ 0.1 mol / L Cu 0.0001 to 0.01 mol / L Other ions SO ₄ ^2- , NH ₄ ⁺ pH 4.5 to 6.5 (adjusted with sulfuric acid and ammonia) Plating conditions: Bath temperature 30 to 40 ° C Immersion time 5 to 50 sec Liquid flow rate 0.06 to 1.40 m / s coating, drying: plating bath _{composition: Ni (en) 3 Cl 2} 0.01 ~0.1 mol / L (en: ethylenediamine) pH 4.5~6.5 ( sulfate, adjusted with ammonia) drying temperature 60 ~120 ℃ [Zn-X alloy electroplating Plating conditions] Plating bath composition: X (sulfate) 0.02 to 1.1 mol / L Zn (ZnSO ₄ ) 0.4 to 0.8 mol / L Na (Na ₂ SO ₄ ) 1 mol / L pH 1.5 to 2.0 (adjusted with sulfuric acid) Plating conditions: Bath temperature 45-50 ° C Current density 50-1 00 A / dm ^Two- fluid flow rate 0.06 to 1.40 m / s Both sides of a Zn-X alloy plated steel sheet that has undergone etching treatment and cracks on the plating film surface are coated with a coating type chromate solution of the following composition using a roll coater. And 150
By baking at ~ 300 ° C to form a chromate film, a sample of the surface-treated steel sheet of the present invention was prepared.

As silica, dry silica (trade name Aerosil 200) having an average primary particle diameter of 7 nm was used. In some tests, wet-process silica (trade name: Snowtex O) having an average primary particle diameter of 10 nm was also used.

[Composition of the chromate treatment liquid] Cr ³⁺ : 50 g / L Cr ⁶⁺ : 1 g / L SiO ₂ : 90 g / L The corrosion resistance of the surface-treated steel sheet thus produced to gasoline and alcohol-containing fuels , External corrosion resistance, and weldability were tested by the following methods. The test results are summarized in Tables 1 and 2.

FIG. 3 is a graph showing a comparison of the fuel corrosion resistance of gasoline and gasohol after processing with the prior art. In the figure, No. 4 in Table 1 was used as an example of the present invention, and a comparative example in which no crack was provided at this time is shown as no crack. Can be

In addition, the turn sheet (Sn / Pb: 0.10, adhesion amount)
At 45g / m ² ), the corrosion of the wall from the shoulder of the punch was large, and the corrosion was particularly easy to progress in the part where the plating was damaged due to the processing.

Test Method [Fuel Corrosion Resistance] A blank (press-punched test piece) of a surface-treated steel sheet was subjected to cylindrical drawing under the following conditions to prepare a cup, and 30 ml of gasoline (gasohol) having the following composition was placed in the cup. Was sealed, and the container was sealed. The fuel corrosion resistance was evaluated based on the following criteria based on the maximum erosion depth (Pm) of the inner surface on the 180th day.

◎: Pm <0.1 mm ○: 0.1 mm ≦ Pm <0.2 mm △: 0.2 mm ≦ Pm <0.5 mm ×: 0.5 mm ≦ Pm Cylindrical drawing condition Blank diameter: 100 mm (diameter) Bunch diameter: 50 mm (diameter , Shoulder r = 5mm) Die diameter: 51mm (diameter, shoulder r = 5mm) Blank holder pressure: 10KN Draw height: 30mm Surface roughness: # 1200 Polished every time Molding without lubricant (degreasing before molding) Degreasing condition 2 % Lid sole immersion (Liquid temperature 53 ℃) 3 minutes → Pure water immersion (
Room temperature) 1.5 minutes → Dry (165 ° C) 8 minutes → Room temperature 20 minutes →
Drying (165 ° C) for 15 minutes (Note) Aggressive methanol
Is 95% anhydrous methanol, 0.1% NaCl + 0.08% Na ₂ SO ₄
+ 5% aqueous solution containing 10% formic acid.

[Outer surface corrosion resistance]
Except that the drawing height was changed to 25 mm, after cylindrical drawing was performed under the same conditions as the above fuel corrosion resistance test, the edges were sealed, and the outer surface was subjected to SST (salt spray test) according to JIS Z2371. For 2000 hours. The corrosion resistance after processing was evaluated by the maximum pit depth (Pm) after 2000 hours of SST.

◎: Pm <0.1 mm ○: 0.1 mm ≦ Pm <0.4 mm △: 0.4 mm ≦ Pm <0.8 mm ×: 0.8 mm ≦ Pm [Weldability] After performing a continuous seam welding test under the following conditions for 100 m , And a cross-sectional microscopic observation of the welded portion was performed and evaluated according to the following criteria.

Seam welding conditions Pressure: 300 kgf Energizing time: 3 cycles Rest time: 2 cycles Current: 13,000 A Speed: 2.5 m / min Weldability evaluation criteria ○: Good welding ×: Blow hole present

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

The surface-treated steel sheet according to the present invention, when used in the manufacture of a fuel tank, exhibits high fuel corrosion resistance not only to gasoline but also to alcohol-containing fuels such as gasohol and the like. It can be manufactured efficiently and inexpensively using the X alloy electroplating apparatus as it is, and is excellent in safety because it does not contain Pb harmful to the human body.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of a cross-sectional configuration of a plating film of a surface-treated steel sheet according to the present invention.

FIG. 2 is a schematic diagram of a surface crack of a plating film.

FIG. 3 is a graph showing the results of examples relating to fuel corrosion resistance after processing of the surface-treated steel sheet according to the present invention and a conventional surface-treated steel sheet.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Eiji Kajiyama 3, Kaji-shi, Kashima-shi, Ibaraki Sumitomo Metal Industries, Ltd. Inside Kashima Works (72) Inventor Yoshihiro Kawanishi 3, Kaji-shi, Kashima-shi, Ibaraki Sumitomo Metal Industries Kajima Works, Ltd. (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 28/00

Claims (1)

(57) [Claims] 1. A first layer of a plating film comprising a plating layer containing at least 70 wt% of Ni having a plating adhesion amount of 0.001 to 5 g / m ^{2 on} one side, and a second layer comprising an alloy element X, Ni: 3
-18% by weight, Co: 0.02-3% by weight, Mn: 25-45% by weight, Cr: 8
At least one selected from the group consisting of ~ 20 wt%,
It has a Zn-X alloy plating film coating weight per one side is 5 ~50g / m ^2, chromate of 10 to 200 mg / m ² of metal Cr equivalent coating weight on top of the Zn-X alloy plating film It consists of a surface-treated steel sheet provided with a film, and the Zn-X alloy plating film under the chromate film has cracks, and the density of these cracks is 1 mm × 1 mm of the area surrounded by cracks in a visual field of 1 mm × 1 mm on the plating surface. The number of cracks is in the range of 1000 to 150,000, and the cracks with a maximum width of 0.5 μm or less are 90% or more, and the crack depth is 80% or more of the plating thickness. Surface treated steel sheet with excellent corrosion resistance after processing.